Writing in a journal about your feelings is a good place to start. Consider it a journey of self-discovery as you search for clues to this mystery. Once you figure out what has led to this emotional detachment from your mother, you may want to talk with a therapist about what you discovered and how to move forward from it.
However, only you can do the groundwork. If you went to therapy without some idea of what caused this indifference, you'd be wasting your time and money. The goal is to become the authority on your own life, history, relationships, and feelings.
You may want to explore the years between birth and age 5 to find answers there. When an early attachment isn't formed between parent and child, a close relationship may never materialize. Was your mom present during those years? If present, was she going through a trauma (a divorce, the death of a parent, the loss of a job, drug or alcohol addiction) that would make her feel depressed, overwhelmed, or too preoccupied to be a loving, involved mom to you as a young child?
If you read my article about emotionally absent mothers because you believe your mom is one, you may have unrealistic expectations. If she has never been a warm, affectionate mommy, it's normal that you wouldn't have those feelings for her. You may be trying to force something that just isn't there.
As the daughter of an emotionally absent mother, I came to practice acceptance. I was creating undue stress for myself by struggling to make things better between my mom and me. Over time, I found peace by accepting the relationship as it was with all its limitations. I now appreciate that she did the best that she could as a parent, especially since she had no role models.
I highly recommend that you read Jasmin Lee Cori's outstanding book, “The Emotionally Absent Mother.” It's said that the person who has created the most psychic pain for us is our greatest teacher. If your mom has caused distress for you over the years, you can take the lessons that you learned and use them to be a better, stronger, and more compassionate human being. Instead of focusing on the relationship with her, you may be better off using your time and energies to foster healthier bonds elsewhere.